1. Field of the Invention
The present invention relates to a system and method for printing an image onto a drum and transferring the printed image from the drum onto a medium. More particularly, the present invention relates to an ink-jet printer with large throughput where an image is printed onto a drum in a helical manner while compensating for skewing and aliasing caused by the helical printing.
2. Description of the Related Art
Ink-jet printers typically use a carriage to move a print head across a medium, such as paper, and to print onto the medium in swaths of defined widths. After each printing pass, the carriage returns the print head to a starting position to begin the next pass, after which the medium is advanced an additional swath width. Eventually, the entire medium is printed onto by the print head. However, time is wasted upon the advancement of the medium and returning the print head to the starting position. This wasted time represents a lower potential throughput. In addition, an objectionable vibration is generated upon returning of the carriage and advancing of the medium, thus generating undesired defects in the resultant printed medium.
Therefore, what is desired is a high speed printing system that provides high resolution with little or no vibration. Recently this need has been met by laser printers. However, the cost of such printers for many business and most home users is too expensive.
The present invention solves this dilemma by introducing a nearly vibration free ink-jet printer whereby an image is printed onto a drum in a helical pattern and therefrom transferred to a medium, thereby increasing throughput. Printing in a helical pattern, however, presents impediments to high image quality. It has been discovered through experimentation that printing in a helical pattern produces skewing and aliasing. Additionally, it has been determined that the drum, the carriage moving the print head, and the nozzles on the print head should all be synchronized.
The skewing produced by printing in the helical pattern can be seen in FIGS. 9 and 11, where, depending on the orientation of the image, an edge of the image is skewed by the width of the print head swath.
Aliasing results when the skewing is corrected. Aliasing shows up as jagged lines that can be objectionable, and is most noticeable on horizontal lines. At a very regular interval, a step appears in the image where one nozzle stops firing and an adjacent nozzle continues, or one nozzle begins firing next to one that is firing continuously.
Conventional drum printers illustrate printing images onto a drum in a helical pattern but fail to address the drawbacks the present invention overcomes.
U.S. Pat. No. 4,293,863 to Davis et al. discloses helical pattern printing. Davis et al. discloses paper being mounted on a continuously rotated drum 12 and a print head 10 mounted on bars and sliding along the axis of rotation of the drum. The print head 10 may be moved in discrete steps or continuously while the drum is rotating. If the print head moves continuously, then the ink pattern is deposited in a helical set of print lines. See Davis et al., at column 6, lines 53–59. While Davis et al. discloses printing in a helical pattern, Davis et al. does not resolve the problem of image skew resulting from helical printing.
U.S. Pat. No. 5,099,256 to Anderson discloses an ink-jet printer depositing ink droplets onto a thermally conductive surface of a rotating intermediate drum. The ink is first deposited directly onto the drum and then transferred to paper. The surface material of the intermediate drum is impervious to ink and enables a 100% transfer of ink to the paper. Anderson requires that the ink be dried through heating the intermediate drum prior to transferring the ink to the paper. However, the intermediate drum does not continuously rotate while the print head is simultaneously moving and ejecting ink, as proposed in Davis et al. Instead the drum rotates a fixed amount with each pass of the print head. This results in wasted time for advancing the printhead, and unfavorable vibration due to the starting and stopping of the printhead upon advancement.
U.S. Pat. No. 5,668,588 to Morizumi et al. discloses a helical light scanning method in which a document is placed on a drum and scanned in a direction parallel to the direction of scan. Morizumi et al. appears to recognize a problem of skewing and proposes computing and adjusting an inclination angle of the light emitting elements to reduce the skewing as the light emitting elements scan across the drum. However, in an ink-jet environment the change in the inclination angle of the carriage must be controlled very closely. If the angle is off by a very small amount, the nozzles in the print head of the various colors will not line up during a single print swath. Further, the next swath starting point will also not line up.
The solution proposed by Morizumi et al. is unlike that of the present invention. The present invention proposes solving the skewing without altering the inclination angle of the print head. Additionally, the use of a ink-jet print head creates additional problems of nozzle placement and selection which are unrelated to the light scanning method of Morizumi et al.
Therefore, what is needed is a simple method and apparatus for helical printing on a rotating drum while simultaneously moving the print head and compensating for skewing and aliasing.